The interaction of objects, whether stationary or mobile, with directed electromagnetic radiation beams has been studied and used in a variety of applications for several decades. One application is a rangefinder, which is a device that measures distance from an observer to an object, for the purposes of surveying, determining the correct focus, accurately aiming, etc. These electromagnetic-based rangefinders can be used to determine the range of an object nearly instantaneously, and with great accuracy.
Directed electromagnetic radiation sources of sufficient power levels can also be used to modify the surfaces of objects upon which they impinge. A variety of physical phenomena occur when a directed electromagnetic radiation source of sufficient power level impinges on an object, including: heating of the surface of the object; formation of a plasma around the surface of the object; ablation of material from the object; and even melting of the object's surface. Obviously, the heating or modification of the surface of an object can affect the internal workings and functionality of the object. Specifically, if the outside surface gets too hot, this heat can be conducted into the interior of the object and render it inoperable.
Reflection of electromagnetic radiation is important for many applications. For example, higher levels of reflection can be used to make measurements on an object using directed electromagnetic radiation beams, such as range finding and inertial measurements, considerably more accurate. Also, highly reflective surfaces can be used to protect the surface of an object from any modification or damaging effects that would result if the impinging electromagnetic radiation were to be absorbed into the surface of an object.
Electromagnetic radiation is a self-propagating energy wave in space and/or through matter. Electromagnetic radiation has an electric and a magnetic field component, which oscillate in phase perpendicular to each other and in the direction of the energy propagation. Electromagnetic radiation is generally classified according to the frequency of the waves, including (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, etc. Of these, radio waves have the longest wavelengths and the lowest frequency and ultraviolet has the shortest wavelengths and highest frequencies. Electromagnetic radiation carries energy and momentum, which may be imparted to an object when it interacts with the matter of the object. The most familiar form of electromagnetic radiation is light, which has a wavelength between approximately 400 nm and 700 nm.
A commonly used device for generating a directed electromagnetic radiation beam in the visible and infrared spectrums is the laser. A laser is a device that emits electromagnetic radiation through a process called Light Amplification by Stimulated Emission of Radiation (LASER). Electromagnetic radiation from a laser is coherent and nearly monochromatic. As a result of these properties of coherence and monochromaticity, a laser can provide an intense amount of electromagnetic radiation power or energy per unit area to the surface of the object the beam is impinging upon.
FIG. 1 illustrates the use of a directed electromagnetic radiation source, in this case a laser 10, which is a device that emits monochromatic and coherent radiation in the form of a laser beam 11, usually at the optical or infrared frequencies. As shown in FIG. 1, the laser beam 11 is impinging on the surface 14 of an object 12 located some distance 13 away from the laser 10. The laser 10 may be one of several possible types, including: gas; chemical; solid state; excimer; dye; free electron; etc. Importantly, although the source of the electromagnetic radiation shown in FIG. 1 is a laser, the present invention can be applied to any form or source of electromagnetic radiation over very large spectrum of wavelengths ranging from wavelengths shorter than ultra-violet, through the visible, near infrared, long wavelength infrared, terahertz wavelengths, to wavelengths longer than millimeter waves.
The laser and object configuration shown in FIG. 1 can be used to measure the distance 13 between the laser 10 and the object 12. Specifically, FIG. 1 illustrates a laser 10 used as a rangefinder, wherein the most common method of measuring distance is performed by sending a laser beam or pulse in a narrow beam 11 towards the object 12 and measuring the time taken by the laser beam or pulse 11 to propagate or travel the distance 13 from the laser 10 to the object 12, as well as the time taken by the laser beam or pulse 11 to reflect off the surface of the object 12 and propagate the distance 13 back to the laser 10. The total time of flight is divided by two multiplied by the speed of the laser radiation to calculate the distance 13. The high speed of light makes inexpensive rangefinders difficult to implement with high (i.e., sub-millimeter) precision. However, other techniques, such as triangulation or multiple frequency phase shift, can be used to significantly improve precision. Furthermore, with modification, the configuration of FIG. 1 can also be used with Doppler methods to determine the speed of the object, if it is moving, as well the direction of the object. Moreover, more advanced techniques allow the acceleration, as well as the rate of rotation of the object to be determined, as well with a reflected electromagnetic radiation beam.
Irregardless of the complexity and resultant precision of the exact laser 10 rangefinder configuration used, an important criteria for the ability of the laser 10 to measure the distance 13 between the laser 10 and object 12 is the reflectance of the object's 12 surface 14. That is, a more reflective surface allows the laser to determine the distance with a higher level of precision and accuracy and also allows the laser to measure the distance 13 when the separation between the laser 10 and the object 12 is a greater distance 13. Therefore, there is a need to create surfaces for objects with higher levels of reflectance so that the objects can be measured more accurately for distance, speed, direction, acceleration, and rate of rotation, as well as other important parameters that can be measured using reflected electromagnetic radiation.
It is also important to note that, while the electromagnetic radiation shown in FIG. 1 is generated by a laser, the source of electromagnetic radiation could be generated by other types of sources and/or operate over any one of a number of wavelengths within the electromagnetic spectrum ranging from below ultra-violet to the above millimeter wavelengths.
FIG. 2 is an illustration of a laser system 15 mounted on a platform 14 in which a laser beam 16 is directed onto the surface 19 of an object 17 moving in free flight above ground level 18. In this case, the object 17 is moving in free flight, either as a projectile or as a self-propelled system. The laser beam 16 is pointed and aimed at the object 17, possibly to measure its distance, as well as other important inertial parameters, and impinges onto surface 19 of the object 17. Again, for the application of range-finding and inertial measurement, it is important for performance and accuracy that the object 17 be as reflective as possible at the interrogation wavelengths. The laser system 15 may employ a tracking system in order to allow the laser beam 16 to continuously impinge onto the surface 19 of the moving object 17 while it is in free flight. This tracking system could be of any known type, such as radar, optical, etc. In fact, the tracking may be enabled by detecting the intensity of the reflected laser beam and comparing it to the emitted laser beam 16.
Consequently, a reflective surface on macro-scale objects, whether stationary or moving, will be useful for applications where the object's distance and inertial parameters are to be measured and possibly continuously monitored.
Furthermore, there is a need for the ability to prevent an impinging electromagnetic radiation beam from heating the object onto which the beam is impinging. It is well known that the interaction of a laser beam with the surface of the object can cause significant heating to the object's surface, if the power of the laser is sufficiently large. This heating can result in a variety of physical phenomena on and around an object, including: heating of the surface of the object; formation of a plasma around the surface of the object; ablation of the material from the object; and even melting of the object's surface.
For example, at higher laser power levels when the beam impinges on a surface, the material surface of an object is heated by the absorbed laser energy and the surface material can evaporate or sublimate. At even higher laser power levels, the material at the surface and the surrounding medium (i.e., air or rarified gas) can be converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam, if the laser intensity is high enough.
In many circumstances, it is desirable that the impinging laser beam result in little or no heating, modification, or damage to the surface of the object, even at moderate to high impinging laser beam power levels. Therefore, there is a need for a method by which an object can reflect most or all of the impinging laser radiation.